The present invention relates to the generation of drops of ink in an ink jet device, and more particularly to the generation of uniform drops from an array of ink jets.
Ink jet printing is one of the methods of producing visible marks for graphic reproduction on a surface which has received considerable research and development effort. In ink jet printing a stream of water-based ink drops (droplets) is projected toward a surface; for example, the surface may be a moving web of paper. The drops are electrically conductive and are electrically charged by charging electrodes near the streams. The drops may be deflected by electrodes in the course of their movement so as to strike the web in a certain location. Alternatively, in a binary-type print/no-print system the drops may be deflected to either strike the surface or be caught by an ink sump (catcher).
It has been suggested that an array of separated streams of drops may be produced from a single ink chamber (manifold). For example, the array may consist of one or more rows of orifices in a chamber in which the ink is maintained under pressure. The ink flows from each of the orifices in a filament and the filament, at a certain point past the face of the orifice, is broken up into individual drops by stimulation (ultrasonic vibration at a fixed frequency). The breaking up of the filament into drops may be effected by an ultrasonic vibratory mechanism attached to, or within, the chamber. For example, magnetostrictive or piezoelectric transducers may provide the necessary vibrations.
It is desirable that the ink jet array, and the electronic circuitry associated with it, be as simple as possible so as to reduce its cost and provide for reliability. It is desirable, to obtain an accurate marking, that each of the jet filaments which exit from the ink chamber should be as uniform in filament width, length, velocity and phase as possible and that each of the tandem series of drops produced from the filaments should be as uniform as possible. More specifically, it is desired that the mass of each of the drops be the same in each stream; that the filaments break up into drops at uniform distances from the orifices; and that the drops each have the same velocity and same phase, i.e., their velocity should be the same at equal distances from the orifice, and that all drops travel the same distances at any given time and all break up from the filament at the same time.
In U.S. Pat. No. 3,739,393 entitled "Apparatus and Method For Generation of Drops Using Bending Waves," which names Richard Lyon and John Robertson as inventors (hereafter the "Lyon-Robertson patent"), an ink jet system for generating an array of drops is shown which attempts to provide uniformity of the filament length of the ink exiting from the single ink chamber. In the Lyon-Robertson patent a chamber containing water-based ink under pressure has an orifice plate having a number of rows of orifices. The orifice plate is excited by a piezoelectric device so that a longitudinal acoustic wave is traveling along its length and is absorbed at the end of the orifice plate. Hence, there is no reflected acoustic wave to form a standing wave. This traveling wave, as it travels from one end of the plate to the other, causes the ink filaments which are exiting from the orifice plate to break up into drops.
Although Lyon-Robertson's teaching provides acoustic energy uniformly throughout the orifice plate, there exists a phase differential of 2.pi./.lambda..multidot..DELTA.x between adjacent jets, where .lambda. is the longitudinal acoustic wavelength and .DELTA.x is the inter-jet spacing. In other words, at any given time there exists a 180.degree. phase reversal for any two jets separated by a distance of .lambda./2. Furthermore, as the acoustic wave travels from one end of the plate to the other, it is attenuated so that the orifices which are further away from the start of the wave receive less acoustic energy than the orifices which are closer to the start of the wave. As a result of the above-mentioned, if we examine the dynamics of droplet generation at any given time, the filament length increases gradually from one end to the other. The location of the first droplet to filament changes by exactly one droplet for every acoustic wave length .lambda. away between jets above the orifice plate.
U.S. Pat. No. 4,138,687 entitled "Apparatus For Producing Multiple Uniform Fluid Filaments And Drops," which names Charles Cha and Shou Hou as inventors (hereafter the "Cha-Hou '687 patent") proposes a system to obtain uniform droplets from an orifice plate in an ink jet system. The Cha-Hou '687 patent discussed the Lyon-Robertson '393 patent and pointed out that its traveling acoustical wave presented difficulties in obtaining uniformity in filament length and did not provide uniformity in drop formation. In the Cha-Hou '687 patent a number of pistons are positioned in the back end of an elongated ink jet chamber having a front orifice plate with rows of orifices. The pistons are vibrationally isolated from the chamber. The pistons generate waves which are transmitted to the orifices through the ink fluid with the chamber. The pistons are excited in the same phase and frequency by a set of piezoelectric devices, so that the filaments would have the same length and generate their drops at the same time.
In the Japanese Patent JOP No. 55-65570, issued May 17, 1980, corresponding to U.S. application No. 958,855, a row array of jet nozzles (orifices) is positioned along an elongated chamber. An inner cylindrical member is of metal and the outer tube wall of the chamber is a piezoelectric material. An annular ink cavity is formed between the metal member and the piezoelectric tube. The vibration of the piezoelectric material is normal to the chamber axis and the resulting periodic pressure waves in the liquid cause the break-up of the ink jet filaments into drops. However, the acoustic energy density is inversely proportional to the distance between the orifice plate (piezoelectric tube) and the center (axis) of the chamber. The energy density E=(r/R)Eo where Eo is the density of the acoustic energy generated at the surface of the tube (cylindrical transducer), r is the radius of the tube, and R is the radius of the ink chamber, i.e., the distance from its center axis to the inner tube wall.